A CD (compact disc) was originally developed as an information storage medium (which will be referred to herein as a “disc”) to record music thereon. Nowadays, however, the CD is also extensively used as an information storage medium for computers. Meanwhile, DVDs (digital versatile discs) and other types of discs, which are developed as high-density multi-purpose media from the outset, are also on the market now. Under the circumstances such as these, there is an increasing demand for a system that can play multiple types of discs with mutually different storage densities (such as CDs and DVDs) by itself. Furthermore, a system that can write data on multiple types of discs with mutually different storage densities has been put on the market recently.
In such a system that can read or write information from/on multiple types of discs, the user picks a type of disc according to the type of information to be read and/or written. For example, as for music information or PC (personal computer) data with a relatively small capacity, a CD-ROM disc, a recordable CD-R disc, or a rewritable CD-RW disc is normally used. On the other hand, as for video information or PC data with a relatively big capacity, a DVD-ROM disc or a rewritable DVD-RAM disc or DVD-RW disc is used. Furthermore, to store high-quality video of a BS digital broadcast, a huge capacity (e.g., 20 GB or more) storage medium, which uses a blue laser beam, has recently been developed as a highly prospective product.
Among various optical disc drives for PCs, optical disc drives that adopt a constant angular velocity (CAV) writing technique, characterized by using different transfer rates for inside and outside portions of the same medium, recently outnumber optical disc drives that adopt a constant linear velocity (CLV) writing technique, characterized by using the same transfer rate at any read location on the disc, in order to increase the highest transfer rate and access performance in CD-ROMs or DVD-ROMs, for example. Such a drive usually performs a playback operation at as high a transfer rate as possible, which is defined by the highest possible performance of the drive, but sometimes cannot perform a playback operation at all due to a scratch or dirt on the information storage side of the disc or the degree of eccentricity or shifted center of mass. To avoid such a situation, the function of changing the rotational velocities and switching the transfer rates according to the disc state has recently been introduced.
Hereinafter, the procedure of retrieving information from an optical disc will be described briefly. First, a laser beam is radiated toward the information storage side of the optical disc and its reflected light is detected at an optical pickup and then amplified by an RF amplifier, thereby generating an RF playback signal representing the quantity of light reflected. The RF playback signal generated in this manner then has its RF noise (which is located outside of its signal frequency band) removed by an equalizer. Also, in the vicinity of its high frequency components that have had their amplitudes decreased due to intersymbol interference, the RF playback signal is boosted. Thereafter, the RF playback signal is digitized to generate read data. Meanwhile, a read clock signal, which is in phase with the read data, is generated from the read data by a PLL (phase-locked loop) circuit. Subsequently, a demodulating section generates decoded information from the read data and read clock signal, and then the information is transferred to a host by way of an interface circuit (such as ATAPI or SCSI).
In this procedure, if the types of discs to play or the play modes thereof are different, then the data transfer rates and the RF frequency bands of the read data are also different. More specifically, the frequency band to be extracted and boosted by the equalizer and the frequency of the clock signal to be generated by the PLL circuit change with the type of the disc to play and the play mode thereof. The read clock signal is determined by the transfer rate and the reference clock frequency as defined by the disc standard (e.g., about 27 MHz for a DVD-ROM). Accordingly, if discs to play or transfer rate modes thereof are changed, then multiple lines of those frequency-dependent circuits are prepared for an optical disc drive and switched depending on specific conditions.
If the read and write operations are carried out by the CLV technique, then the transfer rate is constant and just one line of equalizer and PLL is required. On the other hand, if the read and write operations are carried out by the CAV technique, then the transfer rate changes with a read radial location on the disc. Accordingly, those frequency-dependent circuits need to change their processing reference frequencies according to the read radial location, too.
For these reasons, a conventional CAV optical disc drive that can process multiple types of discs chooses one of a plurality of demodulating sections corresponding to those types of discs during its loading, for example. Then, the optical disc drive divides the disc read locations into a number of radial zones. In reading data from the respective zones, the selected demodulating section sequentially picks one of multiple circuits, associated with the respective transfer rates, after another.
Hereinafter, a conventional optical disc drive that can process multiple types of discs in multiple play modes will be described with reference to the accompanying drawings.
FIG. 24 is a block diagram showing a conventional optical disc. In the following example, a drive that uses a CD and a DVD as two types of discs and can play the DVD in two different playback modes (i.e., high-speed and low-speed modes) will be described. In FIG. 24, three lines of equalizer sections 2003, 2004 and 2005 and PLL sections 2006, 2007 and 2008 are provided. Also, a CD demodulating section 2009 and a DVD demodulating section 2010 are provided as two demodulating sections. These circuits are selectively used by a disc type selector 2001 and a playback mode selector 2002.
In playing a CD, an infrared laser diode 1021 with a wavelength of 780 nm is activated and a predetermined CD light beam 1023 is produced by optical members (not shown) in the optical pickup. On the other hand, in playing a DVD, a red laser diode 1022 with a wavelength of 650 nm is activated and a predetermined DVD light beam (not shown) is produced by optical members (not shown) in the optical pickup.
When these two laser diodes or light beams are switched, the disc type selector 2001 selects the circuits required. For example, in playing a CD, the disc type selector 2001 connects contacts A and C together. Then, an RF playback signal, which has been detected by the optical pickup 103 from the optical disc 102 being rotated by a disc motor 101, is amplified by an RF amplifier 104. The amplified RF playback signal passes the disc type selector 2001 and is input to the equalizer section 2003. The equalizer section 2003 boosts the RF playback signal in a signal frequency band that is suited to playing the CD, thereby generating an RFEQ signal.
Then, a digitizing section 106 digitizes the RFEQ signal, thereby generating read data. From this read data, the PLL section 2006 generates a read clock signal. Using this read data and read clock signal, the CD demodulating section 2009 demodulates the information that is stored on the CD and then transfers the information to a host (not shown).
For example, in playing a DVD at the higher speed, the disc type selector 2001 connects contacts C and B together and the play mode selector 2002 connects contacts D and F together. Then, an RF playback signal, which has been detected by the optical pickup 103 from the optical disc 102 being rotated by the disc motor 101, is amplified by the RF amplifier 104. The amplified RF playback signal passes the disc type selector 2001 and mode selector 2002 and is input to the equalizer section 2005. The equalizer section 2005 boosts the RF playback signal in a signal frequency band that is suited to playing the DVD at the higher speed, thereby generating an RFEQ signal.
Then, the digitizing section 106 digitizes the RFEQ signal, thereby generating read data. From this read data, the PLL section 2008 generates a read clock signal. Using this read data and read clock signal, the CD demodulating section 2010 demodulates the information that is stored on the DVD and then transfers the information to the host (not shown).
For example, in playing a DVD at the lower speed, the disc type selector 2001 connects contacts C and B together and the play mode selector 2002 connects contacts D and E together. Then, an RF playback signal, which has been detected by the optical pickup 103 from the optical disc 102 being rotated by the disc motor 101, is amplified by the RF amplifier 104. The amplified RF playback signal passes the disc type selector 2001 and mode selector 2002 and is input to the equalizer section 2004. The equalizer section 2004 boosts the RF playback signal in a signal frequency band that is suited to playing the DVD at the lower speed, thereby generating an RFEQ signal.
Then, the digitizing section 106 digitizes the RFEQ signal, thereby generating read data. From this read data, the PLL section 2007 generates a read clock signal. Using this read data and read clock signal, the CD demodulating section 2010 demodulates the information that is stored on the DVD and then transfers the information to the host (not shown).
As described above, the conventional optical disc drive that can cope with multiple types of discs and multiple transfer rate modes performs read and write operations on those discs by using at least two lines of equalizer sections, PLL sections and demodulating sections. Accordingly, as the number of disc types or transfer rate modes to be handled increases, the circuit scale increases correspondingly.
Thus, to reduce the circuit scale, a configuration, in which a register circuit is provided for these circuits and variables depending on the disc types and transfer modes are changed by modifying the parameters to be set for the register, may be used.
In that case, however, the parameters setting the operating frequencies of a servo circuit (or servo block) or a PLL section (or PLL block) should be defined for each disc type (e.g., CD or DVD), each transfer rate mode (e.g., 16×, 8×, 4× or 2×) and each disc radial location from which information is retrieved. For that purpose, a setting table area is usually provided in the program area of a system controller so as to store all of those parameters associated with the respective disc types, transfer rate modes and radial locations. In the drive described above, supposing the register to be set has 64 bytes, the setting table area in the program area needs 192 bytes to play a CD and a DVD that is processible at two transfer rates.
In that case, every time the number of types of discs to play and the number of transfer rate modes to be supported increase, the required memory capacity increases significantly, thus raising the cost of the optical disc drive, which is a serious problem to solve.